MINERALS © 253 
Physical properties. These may be investigated 
under the following heads : 
A. Properties depending on the reflection of light. 
CoLouR.—A specimen may show some definite colour, 
é.g., yellow, brown, black ; or it may change in 
colour when it is turned in the light, it is then 
said to be irridescent. 
Quartz and Mica sometimes exhibit this property. 
LustrE.—The lustre of a mineral must be distin- 
guished from its colour; thus Galena or Sulphide 
of Lead is grey in colour, but possesses a metallic 
lustre, whilst Quartz may be colourless or white 
with a glassy lustre. ‘The lustre of a mineral may 
be: 
Metallic—The ordinary lustre of metals. Galena, 
Gold, and Iron Pyrites are examples. 
Glassy—The lustre of glass. Rock-salt and Quartz 
are examples. 
Pearly.—Like a pearl. 
fesinous.—The lustre of resin. Zine-blende and 
Opal are examples. 
Suky.—Like silk. This lustre is confined to minerals 
having a fibrous structure, and is well seen in the 
fibrous forms of Carbonate and Sulphate of Lime. 
Shining.—Having a smooth bright surface. 
Galena and Iron pyrites afford examples. 
B. Properties depending on the transmission of light. A 
mineral may be : 
OPAQUE, when it transmits no light at all—eg., Coal, 
Galena. 
TRANSLUCENT, when it is capable of transmitting light 
but cannot be seen through—e.g., thin pieces of 
Flint. 
